1. Field of the Invention
The invention relates to silver-containing and optionally promoter-containing supported catalysts and catalyst intermediates, processes for their preparation and their use for preparing alkylene oxides by direct oxidation of alkenes using air or oxygen-containing gases.
2. Description of the Prior Art
For the industrial preparation of alkylene oxides, use is nowadays made of the direct oxidation of alkenes using air or using gases containing molecular oxygen in the presence of a silver-containing catalyst. Since alkylene oxides, particularly ethylene oxide, have great economic importance as basic chemicals for numerous downstream products, many attempts have been made to improve the performance of the catalysts used. The numerous proposed modifications for improving the activity and the selectivity concern the support material used, the process for preparing the catalysts and the addition of promoters (R. Landau and R. E. Lidow, "Ethylene and its industrial derivatives" S. A. Miller and Ernest Benn, London 1963; D. J. Hucknall "Selective oxidations of hydrocarbons" Academic Press, London 1974; J. of catalysis 34, 100-114 (1974)).
The operating temperature of a catalyst for preparing alkylene oxides is particularly important. It is desirable to have catalysts which have a high activity and selectivity at a low temperature. A low operating temperature results in, for example, a longer operating life of the catalyst, which is of great importance for the industrial process. Furthermore, the formation of by-products, for example the formation of the isomeric acetaldehyde and of formaldehyde in the preparation of ethylene oxide, is significantly less at lower temperatures and thus makes it easier to remove these impurities during the work-up of the alkylene oxide to give a pure starting chemical which satisfies all requirements.
Apart from the disadvantages indicated above, high operating temperatures often lead to undesired secondary reactions at the outlet of the catalyst bed. The reaction products formed here can damage the performance of the catalyst and also lead to undesirable production stoppages in commercial plants. High operating temperatures also favour the occurrence of uncontrollable hot spots which can cause not only technical malfunctions but also can adversely affect the safety of the preparation process.
The operating temperature of a catalyst can be influenced by the addition of promoters and by the preparation process. Promoters which have been found to be particularly advantageous are added oxides, hydroxides and peroxides of the alkali metals and alkaline earth metals (U.S. Pat. No. 2,404,438). A series of patent applications, for example German Offenlegungsschrift 23 00 512 describes, in particular, the addition of the heavy alkali metals as promoters. In further patent applications, for example German Auslegeschrift 19 20 976 places particular emphasis on barium as promoter among the alkaline earth metals.
German Offenlegungsschrift 27 33 688 claims a process for preparing a silver-containing supported catalyst in which a support material is impregnated with a silver compound, the impregnated particles are activated by at least partial conversion into elemental silver and, finally, at least one of the alkali metals potassium, rubidium and caesium is deposited on the catalyst prepared in this way. The silver-containing impregnation solution preferably contains a barium salt.
All previously described catalysts have the relatively high operating temperature of 230-260.degree. C.
EP-A 38 446 (=U.S. Pat. No. 4,400,308) describes silver-containing supported catalysts in whose preparation a support is impregnated with an Ag-containing lactic acid. After drying, the lactic acid is predecomposed in a defined manner in two stages in an oxygen-free atmosphere and is finally decomposed in an oxygen-containing atmosphere, likewise in a defined manner, during the activation of the catalyst. Such catalysts allow operating temperatures for the preparation of ethylene oxide which at from 160 to 230.degree. C. are significantly lower than those of the prior art; the ethylene oxide selectivities are from 80 to 81%.